Chlorofluorocarbon (CFC) gases, such as refrigerant R12 (CCl.sub.2 F.sub.2), have been used for many years as refrigerants and are especially used in domestic refrigerators and vehicle air conditioning systems. However, there is now considerable concern that these CFC gases are involved in the reduction of stratospheric ozone levels and this has led to proposed restrictions and prohibitions on certain CFCs.
A range of less environmentally damaging replacement refrigerants have been proposed and are being utilised. One such replacement refrigerant which is being used to replace R12 is tetraf luoroethane R134a, (CH.sub.2 F.CF.sub.3) which has properties very similar to R12 in terms of pressure-temperature relationship and refrigerating effect. Provided the correct lubricant is used, R134a is a good substitute for R12 in systems using positive displacement compressors. However, this is not the case in refrigeration systems which employ centrifugal compressors.
In a refrigeration system which employs a centrifugal compressor, the pressure required to condense the refrigerant is produced by dynamic means in a rotating impeller. The rotating impeller produces a pressure proportional to: a) the vapour density of the refrigerant, which is in turn proportional to the molecular weight of the refrigerant; b) the radius of the impeller; and c) the square of the rotational speed.
The molecular weight of R12 is 121 and the molecular weight of R134a is 102. Thus, while the condensing pressures of R12 and R134a are similar, it is clear that a centrifugal compressor designed to operate on R12 will not produce sufficient pressure to condense R134a at the design temperatures required. Typically this has been overcome by changing the impeller dimensions and increasing the rotational speed of the impeller to compensate for the reduced molecular weight of R134a. However this is an expensive procedure.
It is an object of the present invention to provide a refrigerant composition which mitigates the above disadvantage, and provides a refrigerant composition suitable for use as an R12 replacement in a centrifugal compression refrigeration system without substantially changing the impeller or the rotational speed.
Blends of large numbers of refrigerants are disclosed in WO92/01762 but the problems addressed by the present invention are not considered.